Step one, download Pakscape. It's zipping software, you might be able to do it with winzip or such, but i'm not sure how. So download pakscape, search it out on the net.

Next, open pakscape, and open up the .pk3 file of the character you want to use. (say you want to replace kyle with maul, open the pk3 of the maul skin.) Now save that file to your desktop as a Zip file.

Now unzip that file to your desktop, making a new file. That file w/ill probably be called darthmaul or similar. Now open that file, and inside it will be a folder called Models. That is the only one you need, so delete the other ones. Now open the models file, and there will be a file called players. Open that and there will be the file called darthmaul, and thats the only one you need, so delete the others.
Now rename the file that says DarthMaul to Kyle.

Now you need to make a new Compressed file. There should be an option if you rightclick on the desktop and select new-compressed folder.

Once you have your compressed folder, drag the Models file out of the DarthMaul folder into the compressed file.

Now rename your compressed file KyletoMaul.pk3 (you can actually name it whatever you want...but naming it like that lets you know what it does) This should turn it into a pk3 file. Now put that file into your gamedata/base folder and run the game.